The present invention relates to electric solenoids; more particularly, to solenoids which are useful as linear actuators of mechanical apparatus, for example, poppet valves; and most particularly, to an improved proportional solenoid actuator having a higher actuation force over an longer range of travel than a comparable prior art solenoid actuator.
Solenoids are employed as actuators in many different applications. Within the automotive arts, solenoids are known to be used as linear actuators for, among others, braking systems, transmission systems, and gas management valves such as exhaust gas recirculation valves. In many such applications, the need is for a device to operate in an on/off mode, such as to actuate a valve between open and closed positions. In such solenoid applications, high response speed and high actuation force are generally desirable. In other applications, however, such as in metering the flow of gas through a poppet valve, proportional control of the valve pintle may be required, wherein the axial position of the valve head must be highly controlled at any desired location between fully open and fully closed.
Force and stroke of a solenoid are interrelated. Force is an important parameter because it overcomes all loads, including parasitic loads of friction, air resistance, mass (inertia), and return spring. Stroke is important, especially for a valve application, because (in the absence of flow passage limitations) the longer the stroke the greater the permissible flow control range of the valve.
The armature of a solenoid is subject to magnetic forces resulting from the magnetic field formed by the solenoid windings. At any point in the flux field entering or leaving the armature, the flux lines have both axial and radial vector components. In general, radial forces on the armature are parasitic, causing friction, and axial forces are beneficial; in some solenoids, the ratio between parasitic and axial forces may exceed 10:1. However, in proportional control solenoids, sensitive control is achieved by configuring the solenoid such that the pole pieces which magnetically engage the armature surround the armature. Further, the pole pieces may be tapered adjacent to the air gap between them such that magnetic saturation of the pole pieces is progressive as the armature slides within them, enhancing proportional control.
In a well-known configuration of a solenoid-actuated valve, the valve is opened by the action of the solenoid and is closed by a compressed return spring when the solenoid is de-energized. Therefore, the solenoid must overcome the resistance of the spring plus any pressure differential across the valve. In addition, in known solenoid actuators, the solenoid must overcome a phenomenon known in the art as magnetic xe2x80x9cpull-back.xe2x80x9d A solenoid armature is not disposed symmetrically axially within the windings but typically has a xe2x80x9cleading face,xe2x80x9d oriented toward the device to be actuated, recessed within the windings, and a xe2x80x9ctrailing face,xe2x80x9d oriented away from the device to be actuated. In a typical solenoid, when the solenoid is de-energized, the trailing face is at or near the corresponding axial face of the pole piece. When the solenoid is energized, the trailing face of the armature is drawn into the pole pieces. At all stages of axial travel of the armature, the trailing face is subject to axial magnetic flux which creates a parasitic restraining force. The strength of this force is non-linear and varies with the axial position of the armature. This pull-back force must be overcome in any proportional control scheme, and its strength and non-linearity make sensitive proportional control very difficult.
Further, radial parasitic forces are proportional to the armature area subject to radial flux vectors within the pole pieces. The cylindrical surface of the armature and also its leading (axial) face are both subject to such radial vectors.
The practical effect of these phenomena, especially in many automotive applications, has been to limit the use of solenoid actuators to applications requiring stroke lengths substantially less than 5 mm and requiring relatively low actuating forces.
What is needed is a proportionally-controllable solenoid actuator wherein pull-back force and radial parasitic forces are minimized to permit longer stroke lengths and higher actuating forces.
It is a principal object of the present invention to provide a proportionally-controllable solenoid actuator having a long stroke length.
It is a further object of the invention to provide a proportionally-controllable solenoid actuator having a high actuating force.
Briefly described, a proportionally-controllable solenoid actuator in accordance with the invention includes primary and secondary pole pieces that surround an armature axially movable along an axial channel within the pole pieces. The pole pieces are separated axially by an air gap and preferably are conically tapered toward the gap to cause progressive saturation with movement of the armature. The armature is substantially squared off on its leading and trailing faces, orthogonal to its direction of travel, to minimize radial parasitic forces on these faces. Further, the armature is also lengthened such that the trailing face is always outside the axial channel at all points of axial travel, to minimize the pull-back effect of axial flux vectors on the trailing face.